The present invention relates to a grinding machine for grinding the rims of spectacle lenses so as to make the lenses conform to a desired shape.
Grinding machines of this type are known in different configurations. These machines usually comprise a lens holder formed of a divided shaft having a suction cup which can be applied to the optical axis of the spectacle lens blank. The suction cup may be attached to one end of the divided shaft, while the other end of the shaft presses against the spectacle lens blank from the other side. In this way, the blank is held in a relatively secure manner such that its optical axis coincides with the axis of the divided shaft. The spectacle lens blank is defined herein as a spectacle glass lens that is still circular, i.e. has not been made conform to the shape of the opening in a spectacle frame.
The lens holder is supported on a slide or a rocker and may be guided by appropriate movement of the slide or rocker so that the rim of the lens blank rests against a rotating grinding disk. A template corresponding to the opening in the spectacle frame is located on the divided shaft forming the lens holder. When the spectacle lens blank has been ground to the specified dimensions, at a certain angular position, the template abuts against a stop. By means of a slow rotation of the lens holder, the periphery of the spectacle lens blank is ground to the shape desired, which is identical with the template.
Examples of grinding machines of this type are described in German Offenlegungsschrift No. 16 52 019, wherein the rocker is arranged in an approximately vertical position, and in German Gebrauchsmuster No. 72 06 781, wherein the rocker is approximately horizontal. A horizontal rocker is the most prevalent configuration and is advantageous among other reasons, mainly because a uniform grinding pressure is assured simply by the rocker being held on the grinding disk under its own weight. It is a common feature of the known embodiments of this type that they comprise a mechanical copying process wherein a template abutting against a stop must be present.
Attempts have already been made to eliminate the template by scanning the spectacle frame directly (German Pat. No. 12 07 235). This, however, is still mechanical copying.
Electronic circuits have been used in machines of the aforementioned special type only for auxiliary purposes, for example to control the grinding pressure (German Auslegeschrift No. 24 57 285).
The first step away from pure template control is described in German Offenlegungsschrift No. 31 20 632. Here the circumferential configuration of the spectacle lens is presented in a drawing or photographic image and scanned by photoelectronic means to guide the grinding spindle or disk, respectively. In this embodiment, again, the circumferential shape is present in a material form and must be inserted or set up correctly prior to grinding in order to be scanned. A substantial stock of templates or other carriers bodily reproducing the curve must be provided and errors are possible in their handling.